Process for producing powdered light metals,particularly aluminum

ABSTRACT

IMPROVEMENT IN THE KNOWN PROCESS FOR PRODUCING POWDERED METAL BY ATOMIZING A MELT THEREOF WITH STEAM FOLLOWED BY SEPARATING THE POWDER FROM THE STEAM WHICH INCLUDES UTILIZING SUBSTANTIALLY DRY SUPER-HEATED STEAM FOR ATOMIZATION AND PROVIDING ALL SURFACES INTO WHICH THE STEAM COMES IN CONTACT AT A TEMPERATURE SUCH THAT SUBSTANTIALLY NO CONDENSATION OF THE STEAM OCCURS.

Feb. 16, 1971 w. KA RTENBECK 3,564,082 PROCESS FOR PRODUCING POWDERED LIGHT METALS, PARTICULARLY ALUMINUM Filed July 17, 1968 2 Sheets-Sheet 1 ml H mm v

Fig. I

Inventor: MW kmveruas c/ After-nay;

Filed July 17, 1968 Feb; 16 1971 w KARTENBECK 3,564,082

PROCESS FOR PRODUCING POWDERED LIGHT METALS, PARTICULARLY ALUMINUM v 2 Sheets-Sheet 2 In ventor: WaR/VER. k me ea Attov e s Int. Cl. Billj 2/02 US. Cl. 264-12 8 Claims ABSTRACT or THE DISCLOSURE Improvement in the known process for producing powdered metal by atomizing a melt thereof with steam followed by separating the powder from'the' steam which includes utilizing substantially dry super-heated steam for atomization and providing all surfaces'into which the steam comes in contact at a temperature such that substantially no condensation of the steam occurs.

This invention relates to the production of powdered metal. It more particularly refers to an improved process for atomizing molten metal into powder with steam.

Light metal powder, particularly aluminum powder, is usually produced in industry by one of two atomization processes.

In the older process, fused aluminum is atomized by air and the resulting aluminum powder is collected in large dust chambers. This process has the disadvantage that the mixture of aluminum powder and air is substantially always explosive while the powder is being collected. Hardly a year goes by during which no explosion occurs in such atomizing plants. Attempts have been made to avoid or minimize the dangers resulting from such explosive mixture by installing such plants in uncongested areas. Even then, however, the employees required for operating and inspecting such atomization plants are still endangered. In addition to the dangers of such plants, the powder produced thereby is not of sufliciently high quality but has such a wide particle size range that a powder which meets the requirements of most consumers is obtained only if the powder is resolved by screening, whereby substantial amounts of powder are screened off.

The newer of the atomizing processes satisfies these quality requirements to a much greater extent. In this process aluminum powder is produced by atomizing a melt by steam according to the German Pat. No. 949,441. This process results in an aluminum powder which has a desired particle size range. It is stated in the German patent specification, however, that the aluminum powder can be produced by atomization by steam only if the mixture of steam and aluminum powder is fed into cold water immediately after (downstream of) the atomization chamber. This has been considered necessary because steam is rapidly decomposed by aluminum powder into hydrogen and alumina The alumina is highly oxidized and an equivalent amount of hydrogen is formed. Obviously where aluminum powder is desired, alumina is an unwanted side product which is to be avoided and minimized as much as possible.

Aluminum powder which has been collected and suspended in water obviously constitutes no danger at a temperature of 40 C. because the large surplus of water will prevent a temperature rise which would accelerate the United States Patent Office 3,564,082 Patented Feb. 16, 1971 decomposition of the water by the aluminum. However, the water must be separated from the powder and when most of the water has been removed from the aluminum powder there will be an insufiicient quantity of water to suppress or prevent the temperature rise and attendant reaction. Filtration yields a powder which contains 15% water and which may be subjected to local and subsequently extended temperature rises in the vessels used to store the wet aluminum powder before its is dried. These temperature rises may be so high that the mixture of aluminum powder and water starts boiling so that the water is rapidly decomposed, alumina and hydrogen are formed, and a formation of oxygen-hydrogen gas mixtures cannot be avoided.

The drying of the wet aluminum powder which has been collected is also a major large problem. To maintain the formation of oxide on the surface of the aluminum, drying must be carried out in a vacuum. Whereas batch drying can easily'be carried out, such process has the disadvantage that the wet material must be stored with the above-mentioned dangers attendant thereon. Continuous drying involves the problem of how to charge the wet aluminum powder into the vacuum drying equipment and how to discharge the dry aluminum powder from said equipment; this problem has not as yet been solved. Besides, this drying step adds considerably to the operating costs.

'It is therefore an object of this invention to provide an improved process for the production of powdered light metals.

It is another object of this invention to provide improvements in the known steam atomization process for producing powdered aluminum.

Other and additional objects of this invention will become apparent from a consideration of this entire specification, including the claims and drawing hereof.

Understanding of this invention will become apparent from a consideration of the attached drawing in which:

FIG. 1 is a plot of hydrogen formation, by reaction of aluminum and water, against temperature; and

FIG. 2 is a schematic view of the powder formation process of this invention.

In accordance with and fulfilling these objects, one aspect of this invention resides in improvements in the steam atomization of molten aluminum to form powdered aluminum. These improvements include utilizing steam which is substantially dry and which is super-heated, and providing that all surfaces of the apparatus in which the process is carried out which come into contact with steam are maintained at a temperature such that substantially no steam will condense thereon.

It has surprisingly been found that there will be substantially no oxidation of the aluminum by the atomizing steam and substantially no decomposition of the steam if the steam is kept dry and in a superheated state while it is in contact with the aluminum powder. It has also been found that the rate of reaction of dry steam with aluminum powder is very slow and decreases as the temperature increases. This is indicated in the chart of FIG. 1.

The rate of formation is plotted on the ordinate in milliliters H (under standard conditions), per minute, and per square meters of aluminum surface so that the results of the measurement will be independent on the size and shape of the particles. The steep decline of the rate of hydrogen formation with the increase in temperature is particularly beneficial in the atomization process because the steam temperatures are preferably between 200 and 250 C.

If the above-described improved processing conditions are followed, aluminum powder can be produced in a much safer, simpler and less expensive process than is known to the prior art. This process is as follows:

The powder which has been produced by the atomization of a jet of molten metal with superheated steam is conveyed in known manner, with the aid of the steam used to produce the powder, to a separating means in which the powder is separated from the steam. In accordance with this invention, the mixture of metal powder and steam is separated under dry conditions and the mixture of metal powder and steam as well as all equipment parts contacted by said mixture are maintained at temperatures which preclude a condensation of steam. To this end, all portions of the apparatus from the zone in which the metal is atomized to the separating. zone, inclusive, are provided with insulation and/or heating means.

The insulation and/or any heating of the apparatus is mainly for the purpose of preventing the formation of moisture in the equipment so that the formation of substantial amounts of hydrogen or of an explosive gas mixture is prevented. Since dry aluminum powder is produced by the process and the time-consuming batch drying in a vacuum is eliminated, the residence time as well as the amount of aluminum powder stored in the equipment are reduced by about two powers of ten.

It is known that apparatus of the type employed in the process of this invention is not absolutely sealed against the atmosphere. Therefore it has been found to be suitable to proportion the mixture of steam and aluminum powder so as to permit a certain proportion of air to be admixed therewith. To this end, the steam inlet rate and the air exit rate are both measured and the result is used for controlling the addition of preheated air. To minimize the danger of an aluminum-oxygen reaction, the air content of the superheated steam should not exceed 10%. This corresponds to a free oxygen content of 2% by volume so that an aluminum-air explosion will be precluded. To prevent an oxygen-hydrogen gas explosion as well, it has been found desirable that a concentration of 4% hydrogen must not be exceeded downstream of the condenser. This can be insured by adding air to the system at a suitable rate.

Alternatively, the H content of the gas downstream of the condenser may be measured and this information may be used to control the air addition rate so that the formation of an explosive oxygen-hydrogen gas mixture is avoided.

The powder-separating unit is so designed that after atomization of aluminum is stopped, steam continues to be fed with the same free oxygen content until all aluminum powder has been discharged from the separating unit.

Referring now to the drawing, and particularly to FIG. 2 thereof, a schematic view of a plant for carrying out the process is shown. A melting vessel 1 or a tiltable holding furnace for the molten metal, issues a jet of molten metal which fiows into a nozzle 3, which nozzle is mounted on a tower 2 and is surrounded by an annular nozzle 4, through which steam for atomizing the metal enters the tower 2, e.g., at 10 kg./ sq. cm. superatmospheric pressure and 200 C. The metal, such as aluminum, emerges from the nozzle 3 and is atomized by the steam from nozzle 4 and carried by it through the tower 2 and a pipe 5 to a filter 6, where the aluminum powder is separated from the steam. The filter is an enclosed rotary vacuum filter, which admits the mixture of steam and powder throughout its cylindrical surface 7. The residual gas is drawn off at a control head 8. A peeling device 9 continuously removes the powder from the filter surface and drops the powder into a conveyor screw 10. This screw is designed to discharge at both ends into two powder-receiving vessels 11a and 11b.

The powder-receiving vessels can be disconnected from the screw by two valves 12a and 12b and are filled with a dry inert gas before they are filled with powder. When the vessel is bring filled with powder, a preheated inert gas enters the system and displaces the steam which adhered to the powder so that the powder is perfectly dry as it enters the storage vessel. In this way, the formation of an explosive mixture in the powder storage vessels is reliably prevented. The steam, freed from the powder, is passed in a pipe 13 into a condenser 14 and is condensed therein by cooling water, which is sprayed from a shower head 15. The uncondensed gas consists virtually only of air and is drawn off by a pump 16. The capacity of said pum should be selected in view of the rate at which air may be added. The desired steam-air mixture in the system can be exactly adjusted by a gas fiow meter for the exhausted air. The air rate measured by said meter is combined with the measured rate of steam entering through the nozzle.

All parts of the plant where steam might condense, whereby an oxygen-hydrogen gas mixture might form, are provided with a covering or jacket 17 for insulating and/or heating. Heating may be effected, e.g., with dry steam, hot air, electric heating means, or the like. The details of the heating means are not shown because they are conventional and therefore form no part of this invention.

When the plant is started, the powder separator must be heated above 100 C. by the wall-heating means 17 and must be scavenged by the dry atomizing steam. The atomization must not begin until the desired temperature conditions have been established in the system. On the other hand, when the plant is stopped, the system must be scavenged with steam until all of the atomized powder has been discharged.

The following example will serve to illustrate the practice of this invention without being limiting thereon. In this example, reference will be made to the drawing by specific reference characters.

EXAMPLE Molten aluminum was blown into the enclosed chamber 2 of the plant described hereinbefore. The molten aluminum was discharged in a freely falling jet from a melting furnace 1 into a tundish 3, provided with a metering nozzle, and was atomized by superheated steam supplied through an annular nozzle 4 at 10 kg./ sq. cm. and 200 C. Air was admixed with the steam at such a rate that the content of free oxygen was about 1%. The powder was separated in a dry state as described hereinbefore. At a steam rate of 600 kilograms per hour, 100 kilograms aluminum powder was produced per hour. The recovered powder had such a particle size distribution that had a particle size under 40 microns; it had a surface area of 0.86 square meter per gram. The oxygen content of the screened powder was 0.05% 0 corresponding to about 0.1% A1 0 What is claimed is:

1. In a process for producing aluminum powder in a system by atomizing a jet of molten aluminum with impinging steam to form particles, which cool to form said powder, the improvement comprising:

(a) at startup, heating said system above C., and

scavenging said system with dry steam,

(b) atomizing said aluminum jet with impinging steam sufficiently superheated to prevent liquid water formation during said atomization and thereafter in contact with said aluminum,

(c) separating said formed aluminum powder from said still superheated steam by means to provide dry separation thereof, and

(d) at shut down, scavenging said system with steam for removal of atomized powder from said system.

2. Improved process claimed in claim 1, wherein said steam is at about 200 to 250 C.

3. Improved process claimed in claim 1, carried out in an apparatus having walls which are maintained at a temperature such as to substantially avoid condensation of water thereon.

4. Improved process claimed in claim 1, wherein air 8. Improved process claimed in claim 3, including is admixed with said steam. insulating the walls of said apparatus.

5. Improved process claimed in claim 4, wherein said mixture has up to about 2 volume percent oxygen References Cited content. 5 UNITED STATES PATENTS 6. Improved process claimed in claim 1, wherein said 1,156,079 10/1915 Holley 264-12 aluminum powder and said superheated steam are sepa- 1,351,865 9/1920 'Nicol 26412 rated by vacuum filtration. 2,967,351 1/ 1961 Roberts 264-12 7. Improved process claimed in claim 1, including maintaining a hydrogen content in the gas phase of said 10 ROBERT Pnmar}, Exammer process below about 4 volume percent. HALL, Asslstant Examiner 

